This invention relates to quality control for sputtering targets. It relates more particularly to a method of non-destructively testing a sputtering target to obtain information indicative of the thickness uniformity of a layer of material which can be sputtered from the target onto a substrate. Even more particularly the invention relates to quality control of sputtering targets. Sputtering targets suitable for the practice of this invention frequently are of cylindrical configuration having a diameter of about 13 inches and an axial height of about 0.5 inches. Contemporary specifications for such targets call for an ability to sputter coat an 8 inch diameter substrate with a coating having a mean thickness of 1 micron and a standard deviation from that thickness of 0.01 micron or less. This means that at any point along the layer there must be a 63% probability that the thickness will deviate from 1 micron by less than the thickness of 50 atoms. In general, the sputtering process is able to meet such standards if the grain size of the target is sufficiently well controlled.
While it is possible to perform a microscopic examination of a sputtering target and to make qualitative judgements regarding the variation of grain size throughout the target, this is a very slow process. Moreover, the extreme quality requirements demand inspection of every target. Thus, optical inspection is not practical for controlling the quality of commercially produced sputtering targets.
It is well known that the grain size of a crystalline material affects the transmission of ultrasonic waves passing therethrough. Considerable work has been done in this regard in connection with grain size estimation of ferritic steels. Much of that work has involved experimental measurements in the Rayleigh scattering region; meaning that the grain size of the sample material is small compared to the wavelength of the ultrasonic energy. However, relatively little has been done to adapt that technology to the problem of mass production of high quality sputtering targets. Such work as has been done has not found any parameter which can be related to the thickness uniformity of a coating sputtered therefrom. Thus, there has been no effective method of controlling the quality of sputtering targets to assure the required performance.